1. Field of the Invention
The invention relates generally to drill bits which have polycrystalline diamond compact (“PDC”) cutters thereon. More particularly, this invention relates to drill bits which have polycrystalline diamond cutting structures that have a high thermal stability.
2. Background Art
Polycrystalline diamond compact (PDC) cutters have been used in industrial applications including rock drilling and metal machining for many years. In a typical application, a compact of polycrystalline diamond (PCD) (or other superhard material) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting structure produces enhanced properties of wear resistance and hardness, making polycrystalline diamond materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
Conventional PCD includes 85-95% by volume diamond and a balance of the binder material, which is present in PCD within the interstices existing between the bonded diamond grains. Binder materials that are typically used in forming PCD include Group VIII elements, with cobalt (Co) being the most common binder material used.
An example of a rock bit for earth formation drilling using PDC cutters is disclosed in U.S. Pat. No. 5,186,268. FIGS. 1 and 2 from that patent show a rotary drill having a bit body 10. The lower face of the bit body 10 is formed with a plurality of blades 16-25, which extend generally outwardly away from a central longitudinal axis of rotation 15 of the drill bit. A plurality of PDC cutters 26 are disposed side by side along the length of each blade. The number of PDC cutters 26 carried by each blade may vary. The PDC cutters 26 are individually brazed to a stud-like carrier (or substrate), which may be formed from tungsten carbide, and are received and secured within sockets in the respective blade.
A PDC cutter may be formed by placing a cemented carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treated under high pressure/high temperature (HPHT) conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.”
One of the major factors in determining the longevity of PDC cutters is the strength of the bond between the PCD layer and the sintered metal carbide substrate. For example, analyses of the failure mode for drill bits used for earth formation drilling show that in approximately one-third of the cases, bit failure or wear is caused by delamination of the diamond table from the metal carbide surface.
Many prior art PDC cutters have the diamond table deposited on a substrate having a planar interface. However, in an attempt to reduce the incidents of delamination at the PCD/metal carbide interface, several prior art systems have incorporated substrates having a non-planar geometry to form a non-planar interface. U.S. Pat. No. 5,494,477 discloses cutters having a non-planar interface. FIG. 3 illustrates one embodiment of a PDC cutter having a non-planar interface. As shown in FIG. 3, PDC 110 includes a plurality of sloped surfaces 114, 115 between the substrate 111 and the abrasive layer 112.
Additionally, other prior art systems have incorporated an intermediate layer between the diamond layer and the substrate to reduce these stresses. U.S. Pat. No. 5,510,193 discloses an intermediate layer of polycrystalline cubic boron nitride between a PDC layer and a cemented metal carbide support layer. Further, in the '193 patent, the metal binder, i.e., cobalt, is substantially swept from the metal carbide support layer into the intermediate layer and into the PDC layer. The '193 patent contributes the observed physical properties and interlayer bond strengths of the '193 compact to the sweeping through of the cobalt into the intermediate and PDC layers.
Furthermore, an additional factor in determining the longevity of PDC cutters is the heat that is produced at the cutter contact point, specifically at the exposed part of the PCD layer caused by friction between the PCD and the work material. The thermal operating range of PDC cutters is typically 750° C. or less; conventional PCD is stable at temperatures of up to 700-750° C. Temperatures higher than 750° C. may result in permanent damage to and structural failure of the PCD as well as rapid wear of the cutter due to the significant difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. This may result in spalling of the PCD layer, delamination between the PCD and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear, loss of microstructural integrity, and strength loss. This thermal expansion also jeopardizes the bond strength between the diamond table and the carbide substrate.
In order to overcome this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure (either a thin volume or entire tablet) to at least reduce the damage experienced from heating diamond-cobalt composite at different rates upon heating. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, typically nitric acid or combinations of several strong acids (such as nitric and hydrofluoric acid) may be used to treat the diamond table, removing at least a portion of the Co-catalyst from the PCD composite. By leaching out the cobalt, thermally stable polycrystalline (“TSP”) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance. As used herein, the term TSP includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by filling the volume with a secondary material, such by processes known in the art and described in U.S. Pat. No. 5,127,923, which is herein incorporated by reference in its entirety.
Accordingly, there exists a need for thermally stable PDC cutters having a decreased risk of delamination.